Scrubber systems have been developed for controlling harmful emissions from industrial and utility boilers like those used in power plants. Such scrubber systems typically are of either the dry or wet type. Dry scrubber systems generally include an open chamber in which the flue gas is directed through a liquid spray of lime and fly ash slurry. A chemical reaction occurs with the sulfur dioxide in the gas to form a calcium compound in dry particulate form which can then be collected at the outlet of the chamber, thereby "scrubbing" the flue gas free of sulfur dioxide pollutants.
On the other hand, in the so-called "wet scrubbers" the sulfur dioxide is not collected in dry particulate form, but instead is collected in the form of a slurry in a tank of aqueous absorbent for periodic removal in liquid slurry form. In order to avoid excessive maintenance costs from the build-up of deposits of fly ash, the fly ash must first be removed by means of filtration or electrostatic precipitation before the flue gas can be treated in a wet scrubber. This results in two large, expensive structures. Further, since the fly ash is typically removed separately, the alkalinity which otherwise would have been added by the presence of fly ash, is not present, and thus some of the fly ash which was removed must then be added to the aqueous absorbent, which of course adds further expense and complication.
U.S. Pat. No. 3,836,630 shows a system wherein sulfur dioxide absorption and oxidation is carried out in separate vessels.
U.S. Pat. Nos. 4,156,712 and 4,229,417 disclose a method and apparatus for removing sulfur dioxide from flue gases by simultaneous absorption and oxidation in an aqueous absorbent within a single vessel. As the liquid absorbent in the vessel is agitated, flue gas is sparged into the upper region as oxygen-containing gas is sparged into the lower region of the body of liquid absorbent. Alkaline material is introduced into the body of liquid absorbent to fix the sulfur dioxide in the form of a sulfate, which then precipitates out in solution for removal. These references suggest that gas/liquid contacting and solid (fly ash) precipitation can be carried out in the same device, however, since the flue gas is introduced into the upper region of the liquid absorbent via perforate or notched pipes, at least some prescrubbing of the fly ash particulate therefrom is required to avoid clogging.
While effective control of harmful emissions from industrial and utility boilers is a difficult problem at best, this problem can be even more difficult with some types of fuels. For example, refuse derived fuels (RDF) and certain types of coal have relatively high chloride contents. Chlorides are highly soluble in water, and can thus readily be removed from flue gas via a wet scrubber. However, ash and waste solids in a water slurry that contains chlorides present a complicated disposal problem. Standard waste separation techniques such as thickening and vacuum filtration are not effective for chloride and sodium-rich (caustic scrubber) waste from the systems. These highly soluble wastes do not precipitate or settle out readily. One disposal alternative is to redirect the blow down liquid back to the boiler for drying, but this causes boiler operational problems and degrades overall system performance. In addition, even if the wet wastes are directed back to the boiler for evaporation, particulate loadings on the precipitator increase for the wet caustic and wet limestone acid gas removal systems. Further, significant corrosion problems can arise because chlorides react with water to form an acid.
Another alternative is to install an on-site waste water treatment facility, but this is neither physically nor economically attractive.
A need has thus arisen for an improved hybrid flue gas scrubber system of the wet scrubber type which removes chlorides before entering the scrubber, but which does not require pre-removal of the fly ash, so that effective emissions control can be accomplished economically without the expense and complication of special waste water treatment facilities, even with fuels of high chloride content.